The effect of high supersaturation of magnesium carbonate solutions in an ion-exchange bed is displayed on treatment of (Mg2+-Na+) forms of carboxylic resins in the column with solutions of sodium carbonate and sodium bicarbonate. Upon leaving the column the supersaturated eluate decomposes with formation of coarse-grained crystals of nesquegonite : MgCO3 . 3H(2)O. The sorption scheme of isolating pure magnesium compounds from seawater by means of the effect of isothermal supersaturation was advanced and tested on a pilot-plant scale. The proposed process includes stages of isolation of calcium using natural and manufactured zeolites and isolation of magnesium using carboxylic ion-exchange resins. The mathematical model of the dynamics of the ion-exchange process with supersaturation is developed.